Download - Using the past to predict the future: lake sediments and the modelling of limnological disturbance

E L S E V I E R Ecological Modelling 78 (1995) 149-172

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Using the past to predict the future: lake sediments and the modelling of limnological disturbance

N. John Anderson Geobotany Division, Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NV, Denmark

Received 1 July 1993; accepted 30 March 1994

Abstract

Most lakes have been disturbed to varying degrees but for an individual lake the timescale of these disturbances is rarely known. Lake sediments, however, can be used as natural archives of perturbation histories, e.g. acidification and eutrophication. At present the use of simple weighted averaging models permits the reconstruction of a variety of water chemical variables from diatom and other microfossils preserved in lake sediments (pH, total phosphorus, salinity and lakewater temperature). Sediment records can, therefore, provide lake-specific background data for lake management as well as information about their ecological histories. The common models used in palaeolimnology (dating, transfer-functions) are reviewed and their role in environmental monitoring discussed.

Predictions of future lake water quality following lake restoration methods tend to be made from dynamic mathematical models, but they are also used for hindcasting (e.g. the MAGIC model of catchment acidification). A problem with using dynamic models is that they are often site-specific and require calibration for a given lake. Combined with reliable dating, chemical reconstructions from microfossil-based transfer functions offer the possibility of testing hindcast predictions derived from dynamic mathematical models, e.g. for salinity, TP and pH. In this way, sediment microfossil-based models can assist with the parameterization of more complex, dynamic models of contemporary processes. In this review, comparisons between the two approaches (sediment-based and dynamic models) are given and possible future interactions outlined. Validation of mathematical models by palaeolimnological data might enhance their predictive ability when used for forecasting lake recovery. There is clearly, however, a need for a more rigorous approach to palaeolimnology, i.e. critical hypothesis generation. Multidisciplinary studies of lake disturbance, that combine palaeolimnology, dynamic modelling and contemporary process studies, would also be beneficial.

Keywords: Diatoms; Disturbance; History; Paleoclimatic reconstructions; Sediments; Water quality

1. Introduct ion

The importance of long-term temporal perspectives to assist in the understanding of contemporary environmental change and processes is now generally accepted by limnologists (e.g. Kitchell et al., 1988; Reynolds, 1990). Few lakes,

however, have been monitored continuously for long time-periods (for a notable exception, see Talling and Heany, 1988) and, modern experimental work with mesocosms covers only short time-periods (months to a few years). Similarly, whole-lake perturbations, while more realistic in that they involve the response of a number of

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150 N.J. Anderson /Ecological Modelling 78 (1995) 149-172

trophic levels, are normally temporally com- pressed in relation to natural disturbance rates or even anthropogenic perturbation; compare, for example, the timescales of Schindler et al. (1985) with those of Renberg et al. (1993).

The importance of relevant temporal perspectives has meant that lake sediment records are increasingly being used to provide information about recent (e.g. post-1800) rates of change and natural background conditions in lakes (Smol, 1992; Anderson, 1993). It is now possible to reconstruct histories of both primary producers (diatoms, chrysophytes, non-siliceous algae from pigments records) and higher trophic levels (zooplankton, chironomids), together with a number of water chemistry variables (pH, salinity, TP; see below). Importantly, a better understanding of the limitations of the sediment record due to depositional processes, as well as the range of temporal resolution offered by sediment records has meant that palaeolimnology is a substantial tool for monitoring environmental change (Anderson and Battarbee, 1994).

1.1. Ecosystem disturbance

Man has substantially altered his environment over a variety of timescales ( < 101 to > 103 years) and, therefore, palaeoecological methods are increasingly being used to provide information about past-conditions (Schoonmaker and Foster, 1991; Anderson and Battarbee, 1994). Ford (1989) outlined a number of factors that have direct relevance to both defining and understanding ecosystem disturbance: first, a thorough knowl- edge of baseline conditions; second, reliable defi- nitions of natural variability; third, when did the system begin to change; and fourth, the range of possible future trajectories. A temporal component is implicit to all these factors.

Smol (1992) suggested that there are four main ways of determining the four criteria defined above: i. direct historical measurements; ii. space- for-time substitution; iii. hindcasts using dynamic and empirical models; and iv. palaeolimnological reconstructions. As few lakes have been exten- sively monitored, most management decisions have to be based upon the latter three ap-

proaches. Palaeoecological techniques can be used in conjunction with normal space-for-time approaches to provide a more reliable temporal component (Pickett, 1989). Problems of inter-lake variability and regional pollution (which make it more difficult to identify pristine conditions in a given region) suggest that space-for-time approaches will not always provide adequate restoration scenarios, in defining how far a par- ticular lake has shifted from its presumed "natural background" state. Temporally dynamic records of changing environmental situations for a given lake are more useful for management purposes.

The use of empirical models for hindcasting are largely only extensions of the space-for-time approach, because they are based upon contemporary regional relationships (e.g. between total phosphorus and chlorophyll). The use of dynamic models to provide historical reconstructions of chemical variables and/or ecosystem trajectories has become more common in a number of areas of environmental research (climate, acidification, and eutrophication). There have, however, been relatively few attempts to validate these long-term hindcasts. Recent progress in palaeolimnology has now provided means of defining background conditions in lakes, their disturbance histories and temporal scales of natural variability (Anderson and Battarbee, 1994). It has also given modellers a means of independent validation of their hindcast reconstructions.

This paper concentrates on a number of areas related to the use of simple models in palaeolimnology, as well as their relevance to the use and assessment of complex mathematical models in limnology. The first part of the paper deals with simple models in palaeolimnology and their validation by contemporary monitoring data. The second part reviews the role of palaeolimnological records as independent test data for predictions and hindcasts derived from dynamic models. The final section suggests some possible future developments in sediment-based studies. For a description of recent trends and developments in palaeolimnology, see Smol (1990), Battarbee (1991), Anderson (1993) and Anderson and Bat- tarbee (1994).

N.J. Anderson/Ecological Modelling 78 (1995) 149-172 151

2. Models in palaeolimnology

Until recently, palaeolimnology did not use modelling as an aid to the interpretation and understanding of dominant causal processes, as it is often used in contemporary ecology. However, a variety of empirical and statistical models are now commonly used in sediment studies. Prior to a survey of how sediment-studies have and can contribute to the evaluation of the role of complex mathematical models in environmental monitoring studies, some uses of models in palaeolimnology are summarized.

2.1. Dating

Reliable dating of lake sediments is fundamen- tal to the success of palaeolimnology and it is in this area that the most widely used models (in palaeolimnology) are found. Recent (i.e. 100-150 years) sediments are generally dated using 2~°pb (half-life 22.26 yr), except in relatively rare cases where annually-laminated sediments (varves) can be used (Renberg, 1981). 21°pb is a natural ra- dioisotope derived from the decay of 226Ra. In lake sediments, 21°pb is partitioned into a supported (derived from in situ decay of 226Ra) and an unsupported component that is derived from atmospheric inputs via secondary decay from 226Rn (see Oldfield and Appleby, 1984, for de- tails). It is generally assumed that 226Ra and supported 21°pb are in equilibrium, although this is not always the case (Battarbee, 1991).

Due to a variety of factors (e.g. non-uniform sediment accumulation rates and shifting foci of sediment deposition), deriving a chronology is not always a simple function of sediment depth and radioactive decay constants. For many small lakes with catchment disturbance (deforestation, ur- banisation, eutrophication), sediment loads to lakes have increased substantially over recent time-periods, with an associated increase in sediment accumulation rates in the lake basin. These increased sediment accumulation rates result in non-linear relationships between 21°pb concentrations and depth. For these situations, an alternative dating model is necessary - the constant rate of supply model (Appleby and Oldfield,

1978). Now widely used, it to may not be applicable in all lakes (e.g. sediment focusing) and other alternative models or combinations of models must be used to obtain a reasonable chronology (Oldfield and Appleby, 1984). In general, 2~°Pb- dating works well but it is sometimes over-looked that the chronology is to some extent merely a function of the model chosen: independent checks on the derived-chronology are useful (Oldfield and Appleby, 1984).

For many small meso-eutrophic lakes with disturbed catchments, increased sediment accumulation rates pose a more substantial problem for dating than does sediment mixing. However, where benthic invertebrate populations are high or in shallow lakes with extensive wind-induced resuspension (Kristensen et al., 1992; Anderson and Odgaard, 1994), mixing can be problematical. So much so, that mixing parameters are built into dating models (Berner, 1980). When mixing is demonstrably high, and correctly dated, inferences of the onset of perturbation are required, e.g. for atmospheric contamination, deconvolu- tion techniques can be applied (Christensen and Osuna, 1989). In this approach, mixing functions are used to extract the original input signal from mixed, homogenous sediment profiles. Similarly, as a result of post-depositional diagenesis some chemical species are highly mobile within sediments, e.g. sulphur, zinc. To overcome this problem, Holdren et al. (1984) used a one-dimensional, diagcnetic equation to estimate the true onset of sulphur pollution as recorded in the sediments of a small lake in the Adirondack Mountains, New York State, by taking into ac- count post-depositional diffusion and mixing of sulphur.

2. 2. Transfer functions

Much of the relevance of palaeolimnology to studies of recent environmental change has derived from its ability to reconstruct water-chemistry variables via microfossil transfer functions (e.g. pH, salinity, TP). Traditionally, fossil data have been interpreted in a subjective manner using assumptions about ecological tolerances of taxa and their environmental implications (Birks,

152 N.J. Anderson/Ecological Modelling 78 (1995) 149-172

1992). In palaeolimnology, initial attempts to make interpretation more objective centred on the use of multivariate statistics to "classify" sediment assemblages and later to relate them to dominant ecological variables. This approach (analogue matching sensu lato), exemplified - for diatoms - by a number of papers by Brugam (1980; Brugam et al., 1988), was largely derived from a similar approach used in palynology.

The recent development and application of transfer functions in palaeolimnology was largely associated with lake acidification and pH reconstructions (ter Braak and van Dam, 1989; Birks et al., 1990), primarily because of the well-known relationship between diatoms and pH (see Bat- tarbee, 1984, for a review). Transfer functions require an understanding of the present-day distributions and ecological tolerances of the taxa being used, but as modern autecological data are usually lacking except for the most common of species, species-environmental relationships are usually defined by use of surface-sediment training sets. In this approach, microfossil assemblages are enumerated for surface-sediment samples from lakes with modern water chemistry. Such a sampling strategy is inherent whatever statistical method is actually used to formulate the modern microfossil-water chemistry relationship (e.g. linear regression, weighted averaging - see below).

Initial methods of reconstructing pH were largely variants on multiple-linear regression-type models, using either pH preference groups or individual taxa (Battarbee, 1984; Birks et al., 1990). It is now understood, however, that many taxa have non-linear responses to an environmental variable over long gradients, ter Braak and Looman (1986) demonstrated how the response of a taxon to a given environmental variable can be described reasonably well by a Gaussian uni- modal response model. The relationship of individual taxa to the chosen environmental variable, can be fitted by maximum likelihood regression (ML; ter Braak and Looman, 1986; Birks et al., 1990) but this approach is computationally de- manding (Birks et al., 1990). A simpler approach but with similar ecological assumptions, is to use weighted averaging (WA) regression and calibration (ter Braak and Barendregt, 1986). Birks et al.

(1990) demonstrated that WA methods provide similar predictive results (in terms of the stand- ard error of prediction) to ML Gaussian regression and calibration.

Weighted averaging methods of regression and calibration were initially applied to lakewater pH (via both diatom and chrysophyte training sets) but since have been applied to a number of other parameters, including trace metals (Dixit et al., 1991), salinity (Fritz et al., 1991), dissolved organic carbon (Kingston and Birks, 1990) and nutrients (e.g. total phosphorus [TP], soluble reac- tive phosphorus, nitrogen; Hall and Smol, 1992; Anderson et al., 1993; Christie and Smol, 1993). Lake water temperature has also been inferred from chironomid head capsules (Walker et al., 1991).

Surface sediment samples provide a time-in- tegrated sample, covering anywhere between 1 and 5 years, depending on the trophic status of the lake, the sediment accumulation rate and mixing. Although this time-averaged sample in- troduces errors in to the transfer function (especially if the water chemistry is not constant over the same time-period), it also offers the advan- tage of smoothing out inter-annual variability in both biota and water chemistry. There is also a variability component associated with the heterogeneity of both microfossil and bulk sediment deposition patterns (Downing and Rath, 1988; Anderson, 1990). However, in replication studies of diatom and chrysophyte assemblages in surface sediments from low-alkalinity lakes, this error has been shown to be insignificant (Charles et al., 1991). In eutrophic lakes there appears to be greater inherent heterogeneity (Anderson, 1994).

Lack of modern analogues is a factor that may well limit inferences on a Holocene timescale (e.g. Birks et al., 1990). With the rise of global atmospheric pollution, most lakes have altered water chemistry: lakes with little or no nutrient increases or pH reduction may have elevated metal concentrations, even in apparently remote areas. As a result, modern relationships between water chemistry and diatoms derived for N.W. Europe and N. America may not be the same as those in the past. If this assumption is correct, it suggests some degree of evolutionary adaptation,


extinction (replacement of one dominant community by another) or the development of communities adapted to enhanced nutrient, industrially lowered pH a n d / o r elevated trace metal concentrations.

3. Validation of transfer-function water chemistry inferences

As with most model results, microfossil-inferred water chemistry results (predictions) are tested against observed data (Fig. 1). In general, the inferences and observed results tend to be from the same data set, so the calculated error is underestimated because the model is tested against the same data that were used to create it initially. Strictly, the model should be tested against another, independent, data set (Birks, 1985; Birks et al., 1990). There have been relatively few instances where chemistry inferences from training sets are compared with the observed data from an independent test set. An alternative approach is to statistically subsample the training set to estimate the true error, using computer intensive methods such as bootstrap-

ping and jacknife techniques (Crowley, 1992; Birks, 1993a).

Perhaps the best test of transfer functions re- suits, however, is to compare the inferred-results with observed chemistry for sites with long time series of monitoring data. Strictly, such an approach also involves a dating error, as 21°pb chronologies are generally not accurate to + 1 year. However, lakes with high sediment accumulation rates and subsequently lower sediment- water interface mixing can produce reliable and accurate chronologies. Despite these problems there are examples where microfossil chemistry inferences have been matched against long-term records.

3.1. Lakewater acidity

Although there are a large number of lakes for which diatom-inferred pH have been estimated (Charles et al., 1989), relatively few lakes have long-term records for validation (e.g. > 10 year continuous sampling). The reason for this is simple, most acidified or acid-sensitive lakes tend to be in more geographically remote areas or relatively inaccessible, mountainous areas, so routine

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Fig. I. Weighted averaging (WA) inferred lake water chemistry using diatom surface sediment training sets. A: lake salinity (North American Great Plains, Fritz et al., 1991); B: Total phosphorus (gg TP 1 - I ) (British Columbia; Hall and Smol, 1992). These plots of the diatom-inferences (vertical axes) against measured water chemistry (horizontal) clearly illustrate the ability of simple weighted averaging models to reconstruct water chemistry (combined and redrawn from Fritz et al., 1991, and Hall and Smol, 1992).


monitoring was not generally feasible. Further- more, many acidified lakes were actually acidified prior to recognition of the problem, and it was the ability of the diatom-based models to reconstruct these historical pH changes at remote sites that made palaeolimnological methods so important to the debate about the timing of lake acidification and the causal processes (Battarbee, 1990).

The lakes around Sudbury in northern On- tario, however, provided a suitable test for these microfossil models (Fig. 2). The development of the metal smelters in Sudbury resulted in considerable environmental damage to the surroundings due to high levels of sulphur and metal deposition, both the acidification of lakes and loss of terrestrial vegetation (Dixit et al., 1992). The introduction of the superstack in 1972, which reduced local sulphur deposition by putting the emissions higher into the atmosphere, resulted in a rapid and monitored recovery of the lakewater pH at a number of lakes (Dixit et al., 1992). Dixit et al. (1989) demonstrated how the chrysophyte- inferred pH record from the sediments in these lakes clearly recorded both the acidification and its reversal after the introduction of the superstack (Fig. 2). There is also good agreement be-

tween the chrysophyte-model and the observed pH record.

3.2. Salinity

Lakewater salinity is an indirect measure of climate, because lakewater becomes more concentrated as regional climates become drier and lakes change to being closed-basins. Diatom- salinity training sets have been constructed for a number of different geographical areas using varying methods, although most are variants on the weighted averaging model (Gasse and Tekia, 1983; Fritz et al., 1991). The central Great Plains of North America is a particularly important area for climate change research, as it encompasses a particularly sensitive ecotone (e.g. prairie-forest boundary) that has been shown by palynological methods to have shifted during the Holocene (Grimm, 1983). The area is very sensitive climati- cally and hydrologically with the result that many of the lakes in the area oscillate between a dilute (freshwater) state and a chemically-concentrated (saline) state. The major drought of the 1930s resulted in a major reduction in lake-levels in this area, with the result that many lakes changed to closed-basins with associated salinity increases.

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Fig. 3. A comparison of diatom-inferred salinity and measured salinity for Devil's Lake, N. Dakota for the period ~ 1900- 1980. The diatom inferences were made using weighted averaging regression and calibration (WA) (modified and redrawn from Fritz, 1990).

can be very variable in saline lakes, and biases the reconstruction to lower salinities than were actually measured (Fritz, 1990; Fritz et al., 1991).

For the moment, at least, there has not been any verification of the diatom-inferred nutrient models, despite the abundance of sites in north- west Europe with TP records of 25-50 years, e.g. Windermere, Esthwaite, Blelham Tarn in the En- glish Lake District (Tailing and Heaney, 1988; Reynolds, 1990), M~ilaren in central Sweden (WillEn, 1987), and Alpine lakes (e.g. Mondsee; Dokulil and Jagsch, 1992). Given the importance of lake eutrophication and the associated restoration programmes, there is a clear need for diatom-inferred background TP concentrations, both at sites with limited monitoring as well as those sites where the initial perturbation predates monitoring.

4. Validation of dynamic mathematical and empirical models by palaeolimnological data

The long-term record (~ 50 yr) of the salinisa- tion at Devil's Lake was used by Fritz (1990) to test the diatom training set developed for the Northern Great Plains (Fig. 3). Although there is generally good agreement of trends between the diatom inferences and the observed record, there are significant differences between 1900 and 1950 (Fig. 3). This disagreement is attributed to a number of external factors rather than inherent problems with the approach itself or its statistical assumptions. Over such long environmental gradients, individual diatom species also respond, in varying degrees, to other environmental variables (e.g. nutrients and lake levels). Further, erosion and redeposition of older lake sediments (and hence microfossils) as the lake levels change can create erroneous assemblages (with no analogue or real ecological relationships). Reworking of sediment also causes substantial problems for Zl°pb (Engstrom et al., 1989) and results in dating errors that may themselves lead to mismatches between the diatom model and measured salinity (Fig. 3). However, the discrepancies between the diatom-inferred salinity and measured are probably mainly related to diatom dissolution, which

Microfossil-based transfer functions are essen- tially simple, empirical models and, as a result, contrast substantially to the dynamic, mathematical models commonly used in ecosystem modelling. Benndorf et al. (1985, quoted in Ahlgren et al., 1988) outlined a number of factors ("principles") that must be considered before models can be used for environmental planning or deci- sion making. These requirements were first, maximum possible simplicity; second, maximum necessary complexity; third, maximum possible gen- eralization and finally, proper validation.

Attempts to arrive at a balance between suffi- cient mathematical complexity (to increase the ability of the model to mimic reality and hence its relevance to both ecology and lake management) and simplicity (to permit generalizations between lakes) has been difficult to achieve. Most ecosystem models are complex and largely lake specific, and as a result can be difficult to transfer to other lakes (Ahlgren et al., 1988). It has been suggested that models should be more flexible and of a less complex structure so that they can readily be re-calibrated for new sites (Jcrgensen et al., 1986). In contrast, microfossil-based transfer functions


(especially pH) have been shown to be quite general in their application (Birks et al., 1990; Kingston and Birks, 1990). This wide applicability of diatom-based models, in part stems from the cosmopolitan nature of diatoms: there are few endemic species among the dominant diatom species found in Northern America and N.W. Europe. Inter-regional comparisons for diatom- pH training sets indicated only small differences in taxon optima and hence training sets developed for one region can be used in another with a reasonable degree of reliability (Birks et al., 1990).

Ruddiman (1990) summarized the role of palaeoecological data in testing output from Gen- eral Circulation Models (GCMs) for different regions. There is a significant and mutual relationship between GCMs and the fossil record, because GCMs (and dynamic models in general) provide a physical basis (i.e. they are directly process orientated) for interpreting sediment-core records. Equally, where model results show only a poor relationship to the fossil record, the model itself can be re-examined critically and input variables altered if necessary. Ruddiman (1990) specifically was considering GCMs and the COHMAP project (COHMAP, 1988), but his comments are applicable to limnological modelling and palaeolimnology in general.

Transfer-functions that provide reliable and quantitative estimates of lakewater chemistry (pH, nutrients) can be used, therefore, for independent validation of complex mathematical models, particularly where the latter are used for the hindcasting of historical trends (Smol et al., 1991). Aspects of this important role for palaeolimnological records are illustrated here for pH, total phosphorus and cl imate-lake chemistry- groundwater interactions.

4.1. Lake acidification

Parallel to the use of diatom-based pH reconstructions of lake acidification, a number of mathematical models were developed (e.g. IL- WAS, MAGIC) to provide future lake-catchment acidification trends under differing sulphur deposition scenarios. One of the most widely applied

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Fig. 4. Comparison of MAGIC-inferred pH (hindcast for the period 1847-1987) and diatom-inferred pH (* * *) from 21°pb dated-sediment cores for two Scottish lakes (A, Round Loch of Glenhead; B, Loch Doilet). Upper and lower stan- dard errors of prediction for the two models are plotted (modified and redrawn from Jenkins et al., 1990).

models is the MAGIC model (Model of Acidifi- cation of Groundwater in Catchments; Crosby et al., 1985) which, like other dynamic models is site specific and has to be re-calibrated for local groundwater, precipitation chemistry and geological conditions (soil chemistry and weathering rates). MAGIC has been used to help understand catchment acidification processes and to illustrate the extent to which acidification is associated with culturally (i.e. industrially) enhanced sulphur deposition. The model is run from a historical starting point (e.g. 1850) when sulphur deposition is assumed to be lower than that of present (1970-1980) levels (Fig. 4).

There are no quantitative documentary records of sulphur concentrations prior to the 19th century and as a result the choice of sulphur deposition scenarios constitutes a major assumption in the parameterization of the model, with associated implications for the generation of temporal sequences. The model output of surface water and lakewater pH, however, can be contrasted to that derived from the diatom record. This comparison has been done for a number of sites and there is generally good agreement between the


two approaches (Wright et al., 1986; Jenkins et al., 1990; Fig. 4). Where there has been poor agreement the mathematical models can be re- calibrated in an attempt to fit the diatom record more faithfully. Sullivan et al. (1992) suggested the beneficial aspects of using a variety of techniques to fully evaluate the extent and timing of acidification.

4.2. Phosphorus and lake eutrophication

Lake eutrophication is still a major environmental problem for many areas (Rast and Hol- land, 1988). Parallel to the establishment of re- medial strategies, models have been used to understand the basic eutrophication processes and thereby assist restoration measures. Complex models have been assumed to be superior to empirical TP-chlorophyll relationships, because of the high amount of variance unexplained by TP in such relationships (Reynolds, 1987). The inclusion of other nutrients, and biotic (e.g. grazing) factors into the mathematical models helps them to describe better the distribution of algal biomass. Many of the lake-eutrophication models are, in fact, lake specific and/or have different underlying assumptions. There are numerous lake eutrophication models, some of which have been used to estimate historical TP concentrations or loadings (Chapra, 1977). Ahlgren et al. (1988) provide a discussion of the general merits and differences between empirical and the various mathematical eutrophication models currently in use .

Chapra's (1977) phosphorus model for the North American Great Lakes has been used for comparative purposes by Schelske and co-workers in a number of palaeolimnological studies (Schelske et al., 1988; Schelske, 1991; Schelske and Hodell, 1991). There was a poor agreement between the Chapra model estimates of historical epilimnetic-TP concentrations and the accumulation rate of both total geochemical phosphorus and non-apaptite phosphorus for similar time- periods in dated-sediment cores (Schelske et al., 1988). The problems of using geochemical phosphorus profiles as a record of P supply and/or lake water concentrations are well known (En-

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gstrom and Wright, 1984; Anderson et al., 1993). The organic carbon flux profiles do, however, agree quite well with the Chapra model output (Fig. 5; Schelske et al., 1988).

As Schelske et al. (1988) suggested, mathematical models can be useful for independent assessment of interpretations of other palaeolimnological profiles. Although it may difficult to construct, a diatom-phosphorus training set for the Great Lakes to provide estimates of historical epilimnetic-TP concentrations from down-core changes in the diatom assemblages would be useful. Extensive investigations of diatom assemblages in the sediment cores from the Great Lakes clearly indicate non-steady state conditions at 101-102 year temporal scales and responses to European settlement and catchment disturbance (e.g. Stoermer et al., 1985).

For the eutrophication of Lake Ringsj6n (Sverdrup et al., 1991), atmospheric and geologic variables were used as model inputs to provide estimates of the historical, background phosphorus status of the lake. Using a simple mass balance model based on two trophic levels, hindcasts


were modelled, starting in 1730 and the results indicated near constant background TP concentrations of < 20/xg TP 1-1, with a slight increase between 1940 and 1960. These stable background conditions are, in part, a function of the model's simplicity and may not be correct: the southern Swedish landscape (Sk~me) has been fundamen- tally altered over the last thousand years (Berglund, 1990). Moreover, palaeolimnological studies indicate that lakes were not in a steady state over the last two to three hundred years. For example, diatom stratigraphies in Bengtsson and Persson (1978) indicate considerable change in the plankton communities over the last 150-200 years, and hence the nutrient status of the lakes. Clearly, diatom-inferred TP would be a useful guide to the nutrient state of lake Ringsj/Sn prior to the major perturbation in the post-1950 period and as an independent test of the starting inputs to such eutrophication models.

A possible benefit of diatom TP-training sets is that they are generally applicable at the regional level and have acceptable predictive ability (Hall and Smol, 1992; Anderson et al., 1993; Fig. 1B). As such, they can provide reliable TP-reconstructions and hence estimates of background-nutrient concentrations for lakes, over the last 150-200 years, the period of major cultural eutrophication, when there are minimal problems with no- modern analogue situations. However, diatom analysis is slow, requires experienced analysts and can be expensive (due to dating and labour costs). Historical background estimates of nutrient (phosphorus) loading are, therefore, also being based on documentary sources where information about population and agricultural output and fer- tilizer use is taken from census and documentary data and used to calculate lake nutrient loadings, e.g. a combination of mass-balance models and space-for-time approaches (e.g. Persson et al., 1989). A major problem with this approach is that for smaller lakes, the available information may not be specific to a given lake or catchment and it may post-date the initial changes to the system. However, there is a clear possibility to compare the different methods, with palaeolimnological estimates of TP (and the timing of its initial increase) being used to enhance the parameteri-

zation of these simple loading models, as well as more complex models. Importantly, documentary evidence is often lacking for many lakes and covers only the recent past (50-100 years at best), where as sediment records (and dynamic models) offer the possibility of hindcasting over periods of thousands of years (Anderson and Battarbee, 1994).

4.3. Salinity and climate change

The current debate about anthropogenically- induced climate change and "green house" warming is restricted by an understanding of the natural variability of climate (Ruddiman, 1990); it is unclear whether recent climatic changes are part of a natural cycle or not. Because of this lack of long-term records, which can be used to determine natural variation, climate modellers have increasingly looked to the palaeoecological record for evidence of timescales of climate variability. For terrestrial ecosystems, pollen stratigraphies in lake sediments provide a clear indication of climate change during the Holocene, because at relatively coarse temporal and spatial scales, vegetation is a function of climate. However, at finer scales other factors (disturbance, competition) can become more important in determining vegetation composition and hence pollen stratigraphies.

The Holocene pollen record has proved to be invaluable for reconstruction of past climates (Webb, 1984; Davis, 1988), but Holocene palynological records are often used to provide both some of the calibration input to the models and, data for verification (Henderson-Sellers, 1990). This situation can rapidly, therefore, become cir- cular. In this context, diatom-based models of lakewater salinity can provide independent validation of general climate model (GCM) reconstructions of Holocene climate change. Lake sediments have been recognised to contain a valuable record of changing hydrological conditions (Winter and Wright, 1977; Forester, 1987).

Although in many areas, anthropogenic influ- ences on lakes have been dominant for thousands of years, lakes are inextricably linked to meteoro- logical (and climatic) variability at a variety of timescales (Carpenter et al., 1992; Anderson and


Battarbee, 1994). Long-term monitoring has indicated how it is possible to extract climate-related variation from lakes with strong cultural perturbation signals (George et al., 1990). Similarly, Carpenter and Leavitt (1991) were able to filter- out trophic interactions from a high resolution sediment record, and relate residuals to climatic anomalies. For many eutrophic, temperate lakes, however, it may be very difficult to unambigu- ously identify climate-related variability.

This situation is, however, not the case for closed-basin lakes where climate, lake chemistry and lake levels are more clearly related (Street- Perrot and Harrison, 1984; Forester, 1987). The response of an individual lake to changed water- balance (i.e. effective moisture: precipitation- evaporation differences) is a function of both the lake and catchment morphometry, local hydrology as well as the position of the lake within the landscape (Almendinger, 1990). Attempts to understand this relationship between lake-levels and local hydrology are important to help interpret the sediment record in terms of changing moisture balances.

Determination of lake-level fluctuations from lake sediments can be problematical especially in small lake basins (Almquist-Jacobson et al., 1991), although unambiguous evidence exists for a number of sites (e.g. Street-Perrot and Harrison, 1984; Digerfelt, 1988). Salinity, although an indirect record of climate change, provides a clearer link to climatic variability. While the response of individual lakes may not be representative of regional changes in the hydrological regime (Forestor, 1987; Almendinger, 1990; Fritz et al., 1991; Di- gerfelt et al., 1992), analyses of a number of lakes can be synthesised to provide a regional signal for comparison with GCM reconstructions of Holocene climate. The spatial resolution of GCMs is much coarser (500 × 500 km; Henderson- Sellers, 1990) than the spatially-localized signals derived from sediment-records in small lakes. There is, therefore, a need for average, regional signals derived from a number of lakes (Fritz, 1990; Fritz et al., 1991), both to counter the variability of lake-level and salinity changes due to their "landscape position" (Almendinger, 1990) and to match the spatial resolution of the two

approaches. While the spatial component of climate signals of individual lakes may be limited due to their individualistic response to climate change at the catchment scale (Almendinger, 1990; see below), many saline lakes contain considerable amounts of sediment, offering an unri- valled opportunity for the reconstruction of high- temporal resolution climate changes (Fritz et al., 1991, 1994).

4.4. Groundwater modelling

For many lakes, groundwater inputs are important to both their nutrient budgets and their hydrological balance. Almendinger (1990) demonstrated by use of analytical lake and groundwater models that the response of individual lake- levels to changed moisture balances is partly a function of the position of a lake in relation to a river. The model predicts that lakes furthest from a river are more likely to experience a change in their level, as a result of maximal lowering of the regional water table at that position (Fig. 6). This model was tested qualitatively using palaeolimnological evidence of lake-level fluctuations (plant macrofossils, pollen and mineral magnetic stratigraphy) from a number of lakes along a transect away from the Wing River in the Parkers Prairie sand plain in central Minnesota, USA (Fig. 6). The palaeolimnological results provided evidence that supported the model predictions (Digerfelt et al., 1992; Fig. 6) and illustrate the importance of groundwater hydrology for lake levels and their individualistic behaviour.

These data suggested that surface-water hydrology (e.g. lake isolation and change to a closed-basin) is not always the main cause of changed lake levels. Groundwater recharge (N) was more important than evaporation and precipitation (E:P) in controlling lake levels. A1- mendinger (1993) took this approach further by quantifying the changes in N and E:P to repro- duce the observed reduction lake-levels derived from the palaeolimnological data (Digerfelt et al., 1992). The groundwater model was constrained to fit the lake-levels at 7000 BP by sequentially altering N and E:P until the required fit was obtained. The lake levels at 7000 BP were best

160 N.J. Anderson / Ecological Modelling 78 (1995) 149-172

A

River

. N ' . , ' . ' . " . ' - " . ' - ' . - - ' . " - ' . "

' : : ' : : ' : ' : ' " :N " : " : ' : : ' : : ' : ' • . . . . . . H i . . . .

. . . " . • . . . . • . • . N L O . " . . . • . ' . . • . •

~ ' . . ~ River

B m

0-

1

2

3

4

5

6-

7-

Magnitude of lake-level lowering

• Almora lake

• Upper graven lake

O Cora lake

Adley • lake

Distance to Wing River

10 km

Fig. 6. Groundwater changes, lake position in the hydrological landscape and lake-level changes. A. Schematic representa- tion of the influence of lake position on the degree of lake level lowering; stipple represents the interfluvial mound and the hatching the impermeable bedrock. The model predicts that during periods of low groundwater recharge (NLo), the water table (solid line) and, therefore, the lake-level for an individual lake connected to this water table reflects its position relative to the interfluves (rivers); the maximum difference lake level lowering (i.e. the difference between NLO and NHI.) occurs furthest from the rivers. B. Palaeolimnological "test" of part A. Palaeolimnological estimates of lake level' lowering at 7000 years BP for four lakes on the Parkers Prarie sandplain. Lakes were sampled along a transect with increasing distance from the Wing river; maximum lake-level lowering occurred furthest from the Wing River, supporting the predictions of Almendinger's (1990) model (modified and redrawn from Digerfelt et al., 1992).

fitted by the model with a 40-60% reduction in N and E:P reduced to 100-400 mm yr -1. The climatic implications of these changes in the hydrology of the sand-plain were then compared to

palaeoclimatic inferences derived from other methods.

A further example of the possible interaction between dynamic groundwater-climate models and palaeolimnological records is provided by a hydrological model of lake-groundwater interactions at Wabamun Lake in Alberta (Crowe, 1993). The model estimates both lake level and salinity, and for lake-level fluctuations the model results agree well with documentary records. The model has been used predictively to provide future scenarios for lake water quality changes (salinity increases) associated with future climatic warming. A degree of uncertainty surrounds these forecasts, however, because of the limited past- lake level fluctuation and salinity records (~ 40 years) with which the model can be compared. Forty years is probably too short a period to fully encompass natural climate variance in this area when it is coupled with effects due to modern anthropogenic-induced changes. As the sediment record from Lake Wabamun contains a record of Holocene climate change (Hickman et al., 1984), it provides, therefore, suitable tests for the predictive ability of the model. Longer-term hindcasts of lake-level and salinity derived from Crowe's model could be contrasted against diatom-inferred salinity (cf. Fig 3; Fritz, 1990; Fritz et al., 1991), as has been done for pH (above; Fig. 4). The ability of the model to predict past- changes may enhance the accuracy and relevance of the model to predict the effects of future climatic change on lake water quality.

5. Predicting from sediment records

There are now a number of different regions where multiple palaeolimnological studies mean that the results of any future core analyses can be predicted with a reasonable degree of certainty; i.e. cause and effect are reasonably well known. This has also meant that simple conceptual models can be constructed for the response of lakes (of varying types) to different disturbance regimes and over variable timescales (Figs 7 and 8). As these simple conceptual models involve an implicit understanding of the dominant causal pro-


cess(es) involved (sulphur loading, increased TP loading), they can also be used in a predictive manner to try and elucidate what future lake responses might be. It is very difficult to sample every lake in a given region, whatever method- ological approach is being used. Cumming et al. (1992) therefore used a statistically random sub- set of lakes in the Adirondack Mountains of New York State (USA) to predict how many lakes of

the total number present had actually acidified (see Anderson, 1993, for a discussion of this approach). Such generalizations are important for lake management strategies.

An area of palaeolimnology that has not been fully developed is its role in providing analogues for defining rates of response and recovery to lake perturbation, both natural and anthropogenic (Anderson, 1993). Any assessment of the

I

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................................................ f .................... Ill=

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Fragilaria spp.

Cyclotella kuetzingiana, C. comensis, C. comta, C. glomerata, Aulacoseira ambigua, Asterionella formosa

Brachysira vitrea, Aulacoseira distans v. tenella, Tabellana fiocculosa agg., Fragilana virescens v. exigua

Aulacoseira perglabra + v. fiot~niae, A. #rata, Eunotia incisa, Frustulia rhomboides agg.

Peronia fibula, Aulacoseira distans v. nivalis. Tabellaria quadriseptata, Eunotia naegelii

Tabellaria binalis, Eunotia bactriana, Semiorbis hemicyclus, Navicula leptostriata

Achnanthes minutissima, Cymbella microcephala, Synedra acus

i1

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T i m e

(iv)

(11)

011)

2 3 0 0 - 1 0 0 0 B , P .

1 ~ 1 9 6 0 A . D .

~ 1 9 0 0 A . D .

Fig. 7. Species responses to environmental change. Long-term lake ontogeny, natural and industrial acidification, liming and their effect on species succession in Swedish boreal forest lakes. Upper: generalized species succession for S.W. Swedish lakes over the Holocene; Bottom: idealized pH curve for a typical lake in S.W. Sweden. Both plots are based upon analyses of sediment cores from 12 S.W. Swedish lakes (redrawn from Renberg et al., 1993a).


effects of contemporary lake disturbance is com- plicated by the lack of background data with which to compare the recent changes, and the possible complicating effects of other, simultane- ous changes (e.g. nutrient enrichment versus climatic shifts). Sediment records, however, can be used to determine future effects of changed environmental scenarios by selecting periods in the fossil record which may serve as an analogue for future changes (Birks, 1993b), e.g. biotic responses to the Holocene hypsithermal as an analogue for the effects of climatic warming. Impor- tantly, periods in the sediment record can also be selected when man had minimal influence. The response of lake biota to disturbances such as ash-falls or outbreaks of pathogens can provide important information about natural recovery rates (e.g. Hall and Smol, 1993).

5.1. Species trends

The responses of lake communities and water chemistry to acidification and eutrophication are now clearly known, at least in terms of the functional responses of different biota. For example, although the actual species composition may change, acidification generally results in the extinction of planktonic diatoms, and the replacement of a number of benthic, circumneutral taxa (in terms of their pH preference) by acid, toler- ance species (Fig. 7). Lake eutrophication has

similarly well-defined functional responses of the plankton species present, and many of these changes have an autecological basis indicated by both contemporary phytoplankton monitoring and experimental studies (Reynolds, 1984; Sommer, 1990). At a regional scale, the response of lakes to P enrichment is remarkably similar, and the degree of replacement of one species group by another is a sound, albeit subjective, indication of the degree of enrichment (Stoermer, 1988). Initial enrichment may result in the expansion of the species already present (the "more of the same" concept, Reynolds, 1984); typical oligo- mesotrophic species such as Cyclotella and Aula- coseira followed by increases of Asterionella and finally, the rise of Stephanodiscus and Cy- clostephanos species (Fig. 8).

Resource-based competition using nutrient ratios has become central to phytoplankton ecology over the last 15-20 years (Tilman et al., 1982; Kilham and Kilham, 1984). Species succession due to competition for limiting resources can be predicted by Monod kinetics (Sommer, 1988). However, this approach has been criticised because of the inherent equilibrium (steady-state) assumption of the Monod model which, as has been suggested, is unlikely to be fulfilled in the natural environment (Harris, 1986). Sommer (1990, p. 206) argued that this criticism is inappropriate because at multiannual timescales "within-season changes in resource rations be-

# #

~0~ e .:.~#~e ~ ,~'~ .~'~".0~ . ' ~ ~,~ Diatom-inferred Si: P

f f,

Low High Low High RELATIVE ABUNDANCE

Fig. 8. Species responses to environmental change. Lake eutrophication and changed Si:P ratios due to increased P-loads; generalized species succession for Lough Augher (N. Ireland; see Anderson, 1989) illustrating how species replacements occurred as the Si:P ratio dropped with eutrophication; the species trends conform reasonably well to that predicted by the species competition along an Si:P gradient (original).


come undirectional fluctuations around the long- term mean". He further suggested (Sommer, 1990) that species that have similar resource optima (which are often closely related taxonomi- cally; such as araphid species (e.g. Fragilaria, Synedra) and small centric diatoms (e.g. Stephan- odiscus and Cyclostephanos), become dominant if the long-term average nutrient ratios are close to their optima. If resource-based models are appropriate, they should be able to predict species succession, at least in a qualitative fashion (Kilham and Kilham, 1984). As a test, Kilham and Kilham (1984) proposed that the succession of planktonic diatom species recorded in lake sediments might reflect competitive responses to changing Si:P ratios associated with eutrophication. At Shagawa Lake (Minnesota, USA), the fossil record of species succession was similar to that predicted by the model (Bradbury, 1975). Similar species replacements over the last 100- 150 years have been observed at numerous lakes and as such offer additional evidence for the long-term role of nutrient ratios in structuring plankton diatom communities (e.g. Carney, 1982; Anderson, 1989).

Silica budgets of small temperate lakes are unlikely to have been changed by eutrophication (Tailing and Heany, 1988). In small lakes, retention times can be relatively short and there is annual replenishment of Si from the catchment (Stauffer, 1986). Therefore, increased P concentrations will drive the Si:P down over a number of years. However, superimposed over this long-term change in nutrient ratios will be seasonal changes, resulting from variable uptake in the epilimnion by algae and replenishment from the catchment and hypolimnion. The recognition of these scale- dependent responses of phytoplankton to both short- and long-term changed nutrient ratios helps, to some extent, to reconcile the differences between equi l ibr ium/non-equi l ibr ium approaches to phytoplankton ecology (Sommer, 1990).

5.2. Model building with stratigraphic data

Associated with the increasing number of regional, multiple-lake palaeolimnological studies,

is the possibility to build simple models from stratigraphic data. This approach is complemen- tary to, and often implemented via constrained ordination techniques (see below). These simple, regression-type models can be used to model and hence predict one stratigraphic variable from other stratigraphic variables. Renberg et al. (1993b) used forward selection within canonical correspondence analysis and constrained multiple regression of stratigraphic data (e.g. cultural pollen indicators, charcoal) from 6 lakes in South West Sweden to identify significant predictor variables for diatom-inferred pH (the response variable). Single variable and composite (two variable) models explained between 60 and 95% of the variance in pH and indicated the extent to which prehistoric land-use (burning and grazing) influenced lakewater pH (Renberg et al., 1993b).

Such relationships between stratigraphic variables may have some implicit ecological basis. Korsman et al. (1994) reported strong relationships (Fig. 9) between the arrival of spruce (Picea abies) in boreal forest catchments (as indicated by its pollen record) and changes in the diatom communities. Diatom-inferred dissolved organic carbon (DOC) increased above background levels with the arrival of spruce at three lakes, due to the build-up of humus within the catchment and the increased humic content of surface waters. At these lakes there were significant correlations between the pollen predictor variable (spruce pollen) and diatom assemblage responses as determined by redundancy analysis (RDA; Korsman et al., 1994). The affinity of a number of diatom species for high DOC concentrations has been documented (Korsman et al., 1994, for references).

Importantly, the significance of the relationships between two stratigraphic datasets can be tested by Monte-Carlo permutation type tests (Birks, 1992, 1993a,b), or by using a model developed at one lake to predict the response variable at another lake. For example, Odgaard (1992) developed a simple model based on RDA of the relationship between charred particles (associated with catchment burning) and Calluna pollen. The relationship at one lake (Sols¢) was suffi- ciently good that charred particles could be pre-


0rvattnet (RDA-analysis)

.~6.

(a)

PICEA

A~

- 3 - -

-3 ; ~ ; axis 1 (X=0.35)

Fig. 9. Diatom-vegetation relationships illustrated by constrained ordination (RDA - redundancy analysis) of two sets of stratigraphic variables (pollen and diatoms) for a small S.W. Swedish lake, ()rvattnet (see Korsrnan et al., 1994). (a): a biplot of samples (diatom assemblages, i.e. the response variables) and environmental indicators (tree pollen assemblages, the predictors) showing how the diatoms and by association, the lake chemistry, changed in response to the immigration of spruce into the catchment. The sample numbers indicate the direction of the change: 1 being the youngest, 50 the oldest. (b): diatom species (©), pollen relationships showing the strong relationship of a number of diatoms (e.g. Aulacoseira tenella: Au tene; A. subarcfica: Au sub2; Tabellaria floculossa: Ta floc.) with tile arrival of spruce, as indicated by their strong association with the spruce vector (original; data from Korsman et al., 1994).

dicted reasonably well at a nearby lake, Sk[ins¢~. The possibilities of constrained ordination/regression techniques and the subsequent testing of the relationships via permutation tests has not been fully explored in palaeolimnology (Birks, 1992, 1993a,b).

5.3. Acidification and critical loads

Although the causal processes of surface water acidification have been identified, a major re-

maining hurdle to lake recovery is the reduction of acidic deposition to levels where the lake will, in theory, recover fully (i.e. without assistance from, e.g. liming). This critical load differs from lake to lake and is a function of the acid-neutral- izing capacity of the lake and its catchment. It can be derived from both empirical (Battarbee, 1992; Henriksen et al., 1992) and dynamic models (e.g. MAGIC; Cosby et al., 1985)

Dynamic models, such as MAGIC are important because they are time dependent and can be


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used to indicate how rapidly future targets might be met (Warfinge et al., 1992). However, because of the difficulties in parameterization of the model for individual sites, it is not suited for national mapping exercises. Empirical models are more appropriate for mapping. Of the empirical models, the diatom model (Battarbee, 1992) is most useful for setting the site or basic critical load, whereas the steady-state chemistry method is used for setting critical loads to protect species or species groups.

The diatom-model uses palaeolimnological data and is based on the assumption that a lake passed its critical load when the first signs of acidification occur in the diatom record. Al- though the historical sulphur loading is unknown,

an empirical relationship can be derived by relat- ing the distinction between acidified and non- acidified sites to the present day Ca 2÷ concentration in the water column, as a measure of sensitivity, and sulphur deposition (Fig. 10). This critical ratio for the Ca:S relationship of 94:1 is then used to calculate the critical load for individual sites. These values can be mapped, and by sub- traction from measured sulphur deposition, can be used to calculate and map critical load ex- ceedances (Battarbee, 1992). Importantly, the diatom-model (Fig. 10) can be tested at any lake site by analysis of sediment cores. It is possible, therefore, to use the model as a simple predictive tool to estimate the extent to which lakes will have acidified in a given area, and the number

166 N.J. Anderson / Ecological Modelling 78 (1995) 149-172

200

150

100

(..)

o ° ) / o / O • • o2 o :?

50- / . . . . . . = / o . - - . ; • , * ; • - * "."

0 ' i 0.0 1.0 2 0

S deposition (Keq H* ha -1 yr -1)

3.0

Fig. 10. Plot of calcium concentration against sulphur deposition for low alkalinity lakes in the United Kingdom. For each site diatom analysis of sediment cores has been carried out; open circles represent sites for which there is no diatom evidence of acidification, solid circles show acidified lakes. The Ca:S ratio of 94:1 is determined by logistic regression of the data and indicates the conditions that best predict whether a lake is acidified or not. The ratio can be used to derive an empirical model to calculate "critical loads" for sulphur for other sites (Battarbee, 1992). Unpublished diagram reproduced by permission of R.W. Battarbee.

that can be restored following a given reduction in sulphur loads.

6. Future interactions between palaeolimnology and dynamic modelling

As computer processing power has expanded, the use of models has become increasingly routine in limnology, and recently palaeolimnological studies have also started to make use of these computing changes (Birks, 1993a). There are three, interrelated areas where palaeolimnological studies could positively interact with contemporary limnology and modelling studies (cf. Frost et al., 1988): effects of climate and lake morphometry on productivity (and its change over time), sediment deposition models and changed sediment deposition patterns.

Apart from a number of direct effects on aquatic ecosystems (temperature effects on or- ganisms and productivity; Carpenter et al., 1992), there are also indirect effects of climate change

associated with lake thermal structure and hydro- dynamics, as well as the salinity changes associated with moisture balance fluctuations, discussed above. Lake morphometry can exert an important control on lake productivity (Fee, 1979; Carpenter, 1983), and over Holocene timescales lake morphometry evolves as sediment accumu- lates within the basin (Davis et al., 1986). If these morphometric changes are coupled with climatic changes (also at 103 year scales), there will clearly be interrelated effects on lakewater stratification, metalimnetic stability and lake productivity (Davis, 1992). The effects of increased temperatures on water-column stability have been modelled under changing climatic scenarios (Hondzo and Stefan, 1993). If increased air temperatures lead to enhanced column stability and meromixis (Smol et al., 1991), the main location of primary production may shift from the epilimnion to the metalimnion. There are lake sediment analogues for such changes. Increased water column stability may be reflected in pigment records of metal- immnetic bacteria, cyanobacteria and chryso-

N.J. Anderson / Ecological Modelling 78 (1995) 149-172 167

phytes scales (Fritz, 1989; Leavitt et al., 1989). General productivity shifts can be inferred from microfossil records (via weighted averaging inferences to TP, chlorophyll) and water temperatures from chironomids (Walker et al., 1991). Modelled changes in lake productivity associated with increased lake temperatures and water column stability (e.g. Hondzo and Stefan, 1993) could then be "tested" against the palaeolimnological record.

The Holocene hypsithermal period, a time even in North-West Europe, when the effects of early agriculture were minimal, provides a suitable "test period" for the hypothesis that future climatic change will result in increased lake productivity. Many lakes would have been deeper at this time, and liable to more stable stratification. Northern boreal forest systems, where man's impact has been minimal until relatively recently, coupled with the sensitivity of these regions to climatic shifts (Schindler et al., 1990), provide another suitable testing ground for climate-lake productivity interactions.

The influence of lake morphometry on a number of limnological parameters has been modelled theoretically (Carpenter, 1983). The importance of sediment focusing for the interpretation of sediment and microfossil accumulation rates led to one of the first attempts to use models in palaeolimnology (Lehman, 1975). Unfortunately, however, the correction factors suggested have proved to be inappropriate, as sediment accumulation patterns are more complex and, moreover, change in ways not only related to basin morphometry (e.g. dominant wind stress; Odgaard, 1993). Simple conceptual models now exist for the dominant sediment deposition processes in lakes (Hilton, 1985), and sediment resuspension can be predicted with reasonable accuracy by both empirical and mathematical models (Blom et al., 1992; Evans and H~kanson, 1992). As these dynamic models are often concerned with the effect of resuspension on nutrient concentrations and/or organic pollutants, they do not generally include explicit statements as to where the resus- pended sediment will be redeposited. However, the sediment record provides a long-term record of these changing foci of redeposition (Davis et al., 1984). Moreover, in some lake basins the

nature of the sediment accumulation pattern appears to reflect dominant wind directions (Odgaard, 1993) offering the possibility of calculation of palaeo-fetches.

Recent studies have demonstrated how sedimentary metal concentrations in lakes can be predicted from simple models including catchment and morphometric variables (Rowan and Kalff, 1993), and whole-basin sediment fluxes can be used predictively to separate atmospheric and catchment pollution burdens of trace metals (Blais and Kalff, 1993). An important requirement for future sediment-based studies will be simple predictive models which can be used to reduce the number of cores that need to be taken from a lake to accurately estimate the whole-basin sediment flux. After suitable validation on a range of multi-cored lakes with mapped 3-dimensional sediment distributions, there might be the possibility to predict for different lake types and geo- graphic regions - in an approximate way - where the dominant centres of sediment accumulation are (and were) prior to actually coring a lake. Such a scenario would permit a more rigorous approach to lake sampling strategies and the interpretation of accumulation rate data, one of the aims of the sediment-focusing model (Lehman, 1975; Davis et al., 1984).

7. Conclusions

Palaeolimnology has been and still is, largely a descriptive science, because the data type (i.e. qualitative), its inherent historical nature, together with subjective analysis do not lend themselves to experimental/deductive approaches (Bi- rks, 1985, 1992, 1993b). While this does not nec- essarily reduce its relevance to environmental disturbance or ecological problems (because of the temporal component), this implicit qualitative aspect of the data has, until recently, resulted in neolimnologists assuming that palaeolimnology had little relevance to contemporary problems and process studies. The benefits of the long-term perspective are now self-evident (Frost et al., 1988; Kitchell et al., 1988; Reynolds, 1990). Al- though palaeolimnology has become more quanti-


tative and objective over the last decade, there is still considerable room for improvement (Birks, 1985, 1992, 1993b; Anderson, 1993).

It is actually very difficult to approach sediment-based studies in any other way but induc- tively (Birks, 1992, 1993b). However, where multiple variables have been analysed from sediment cores, interpretation can be easier and perhaps more realistic (due to interpretations being supported by each other), although the danger of circularity in the interpretations is often over- looked (because of the strong degree of interde- pendence in stratigraphic data). It is in this area that the use of constrained ordination techniques to partial out strong and autocorrelative (i.e. time) effects in stratigraphic data is only now being explored (Birks, 1992, 1993a,b). Recent improve- ments in both computer power and software have meant that it is now relatively simple to approach data analysis and interpretation more rigorously. Even these statistical approaches cannot, however, substitute for sound reasoning in the initial planning of palaeolimnological projects (Birks, 1985). However elegant an interpretation, the inability to refute a badly constructed working hypothesis may limit its wider relevance.

Dearing (1991) suggested that greater "empha- sis should be given to how lake sediment data may be used deductively to test models or hypotheses" (p. 104). As indicated above, dynamic models could be developed for a variety of long- term scenarios and then tested against long-term, sediment-based records. Computer-intensive resampling techniques (bootstrapping, jacknifing) and Monte-Carlo permutation tests now provide palaeolimnologists with greater possibilities for critical evaluation of their own data (Birks, 1992, 1993a,b). Combined with dynamic modelling and hypothesis generation and, using the clear oppor- tunities offered by the sediment record of a variety of timescales and disturbances (Anderson and Battarbee, 1994) it should be possible to create and test hypotheses of the dominant causal processes in long-term environmental change and their effect on aquatic communities.

Simple weighted averaging models can now be used for reconstruction of a variety of water chemical (and other) variables, thereby strength-

ening the possibility of interaction between palaeolimnological and dynamic modelling approaches. For example, Frost et al. (1988) concluded that a combination of simulation models and lake sediment records offered the best possible way of determining a natural baseline and its variability for a lake. Dynamic models are powerful predictive tools, and it this temporal component that lends itself to greater interaction with palaeolimnological records, because of the inherent time aspect of lake sediments. Dynamic models are generally site specific while microfossil-based transfer functions are usually applicable at the regional scale. Lake sediments offer the possibility of validation of hindcasts derived from dynamic models. While there are clearly many op- portunities for the interaction of sediment-based studies and modelling, palaeolimnology itself must also attempt to increase its objectivity, and hence its relevance to environmental issues.

Acknowledgements

I am grateful to Eugen Rott and Eveline Pipp for the invitation to present this review at the Innsbruck workshop. I would like to thank Helle Zetterwall and Tom Korsman for help with the figures, and to Rick Battarbee for permitting me to use his critical loads figure.

References

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